diff --git a/default_native_helper_darwin.go b/default_native_helper_darwin.go
index abb158d..1a9aca6 100644
--- a/default_native_helper_darwin.go
+++ b/default_native_helper_darwin.go
@@ -17,6 +17,7 @@ package credentials
 
 // getPlatformDefaultHelperSuffix returns the platform default credential
 // helper suffix.
+// Reference: https://docs.docker.com/engine/reference/commandline/login/#default-behavior
 func getPlatformDefaultHelperSuffix() string {
 	return "osxkeychain"
 }
diff --git a/default_native_helper_linux.go b/default_native_helper_linux.go
index cd5de94..f182923 100644
--- a/default_native_helper_linux.go
+++ b/default_native_helper_linux.go
@@ -19,6 +19,7 @@ import "os/exec"
 
 // getPlatformDefaultHelperSuffix returns the platform default credential
 // helper suffix.
+// Reference: https://docs.docker.com/engine/reference/commandline/login/#default-behavior
 func getPlatformDefaultHelperSuffix() string {
 	if _, err := exec.LookPath("pass"); err == nil {
 		return "pass"
diff --git a/default_native_helper_unsupported.go b/default_native_helper_unsupported.go
index 7403d87..c506b84 100644
--- a/default_native_helper_unsupported.go
+++ b/default_native_helper_unsupported.go
@@ -20,6 +20,7 @@ package credentials
 
 // getPlatformDefaultHelperSuffix returns the platform default credential
 // helper suffix.
+// Reference: https://docs.docker.com/engine/reference/commandline/login/#default-behavior
 func getPlatformDefaultHelperSuffix() string {
 	return ""
 }
diff --git a/default_native_helper_windows.go b/default_native_helper_windows.go
index b97135f..e334cc7 100644
--- a/default_native_helper_windows.go
+++ b/default_native_helper_windows.go
@@ -17,6 +17,7 @@ package credentials
 
 // getPlatformDefaultHelperSuffix returns the platform default credential
 // helper suffix.
+// Reference: https://docs.docker.com/engine/reference/commandline/login/#default-behavior
 func getPlatformDefaultHelperSuffix() string {
 	return "wincred"
 }
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index db732cb..040287e 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -1,3 +1,18 @@
+/*
+Copyright The ORAS Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
 package config
 
 import "testing"